The defense mechanism of Limulus Polyphemus against invading microorganisms has been one of the main interests of our laboratory. In the current studies we have isolated a number of proteins and peptides with antibacterial and/or with agglutinating properties. The purification procedures using affinity chromatography with surface components of bacteria such as lipopolysaccharide, resulted in the isolation and characterization of a 12 kD protein which has the ability to bind to Limulus blood cells and to both gram positive and gram negative bacteria. We have been studying the nature and specificity of such interactions and exploring the lectin properties of this and other components isolated from Limulus hemolymph. Partial sequence data of other agglutinins has been obtained and the full characterization of these proteins is the object of our present investigations.